In JavaFX, you can use a Timeline to create a timer. The Timeline class provides a way to create animations and timed sequences of events. By using a KeyFrame, you can specify a specific point in time when an event should occur.

Here's an example of how to create a timer in JavaFX using a Timeline:

import javafx.animation.Animation;
import javafx.animation.KeyFrame;
import javafx.animation.Timeline;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
import javafx.util.Duration;

public class TimerExample extends Application {

    private int seconds = 0;

    @Override
    public void start(Stage primaryStage) {
        // Create a label to display the elapsed time
        Label timerLabel = new Label("Elapsed Time: " + seconds + " seconds");

        // Create a Timeline with a KeyFrame that updates the label every second
        Timeline timeline = new Timeline(new KeyFrame(Duration.seconds(1), event -> {
            seconds++;
            timerLabel.setText("Elapsed Time: " + seconds + " seconds");
        }));

        timeline.setCycleCount(Animation.INDEFINITE);

        // Create a StackPane layout and add the timer label to it
        StackPane layout = new StackPane(timerLabel);

        // Create a Scene with the layout and set it on the stage
        Scene scene = new Scene(layout, 300, 200);
        primaryStage.setScene(scene);

        // Start the timer
        timeline.play();

        // Show the stage
        primaryStage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
}

In this example, we create a Timeline that updates a label every second to display the elapsed time. We set the cycleCount of the Timeline to Animation.INDEFINITE so that it will continue running indefinitely. We create a StackPane layout and add the timer label to it. We then create a Scene with the layout and set it on the stage. Finally, we start the timer by calling the play() method on the Timeline.

You can customize the timer by adjusting the Duration of the KeyFrame and the behavior of the timer using the various methods of the Timeline class, such as setCycleCount() and setAutoReverse().